One of conventional hydraulic drive systems for construction machines which can simultaneously drive a plurality of actuators is disclosed in JP, A, 2-248705. The disclosed hydraulic drive system comprises first and second hydraulic pumps, first and second actuators driven by a hydraulic fluid supplied from the first and second hydraulic pumps, and first and second valve apparatus respectively disposed between the first and second hydraulic pumps and the first and second actuators for selectively controlling operation of the first and second actuators. The first valve apparatus includes a first flow control valve and a first directional control valve cooperating each other, and a first pressure control valve disposed between the first flow control valve and the first directional control valve. The second valve apparatus includes second and third flow control valves and a second directional control valve cooperating one another, and a second pressure control valve disposed between the second and third flow control valves and the second directional control valve. The first hydraulic pump is connected to the first actuator via the first flow control valve, the first pressure control valve and the first directional control valve, as well as to the second actuator via the second flow control valve, the second pressure control valve and the second directional control valve in parallel to the first actuator. The second hydraulic pump is solely connected to the second actuator via the third flow control valve, the second pressure control valve and the second directional control valve. With such an arrangement, only the hydraulic fluid delivered from the first hydraulic pump is supplied to the first actuator, while the hydraulic fluid delivered from the first hydraulic pump and the hydraulic fluid delivered from the second hydraulic pump are joined and then supplied to the second actuator. The hydraulic fluid delivered from the first hydraulic pump and the hydraulic fluid delivered from the second hydraulic pump are joined between the second and third flow control valves and the second pressure control valve.
The above hydraulic drive system also comprises a pressure signal transmitting line for introducing higher one of load pressures of the first and second actuators, as a pressure signal, to drive sectors of the first and second pressure control valves. In response to the pressure signal, the first and second pressure control valves operate in valve-closing directions such that the first pressure control valve controls a pressure downstream of the first flow control valve and the second pressure control valve controls a pressure downstream of the second and third flow control valves.
Further, the above hydraulic drive system comprises first and second pump regulators for controlling delivery rates of the first and second hydraulic pumps, respectively. The first and second pump regulators are supplied with, as a pressure signal, higher one of the load pressures of the first and second actuators via the aforesaid pressure signal transmitting line to control the delivery rates of the first and second hydraulic pumps so that delivery pressures of the first and second hydraulic pumps are held higher than the pressure signal.
In the hydraulic drive system thus constructed, the combined operation of the first and second actuators can be surely performed even when the load pressures of the first and second actuators are different from each other. For example, when the first actuator is driven at 200 bar on the higher load pressure side and the second actuator is driven at 100 bar on the lower load pressure side, the higher load pressure of 200 bar is introduced to the pressure signal transmitting line. Therefore, the first and second pump regulators cause the delivery pressures of the first and second hydraulic pumps to be held at a pressure, e.g., 220 bar, higher a fixed value than 200 bar. At this time, the pressure of 200 bar is also introduced to the drive sectors of the first and second pressure control valves via the pressure signal transmitting line so that pressures upstream of the first and second pressure control valves, i.e., pressures downstream of the first flow control valve and the second and third flow control valves, are held at 200 bar. Thus, since pressures upstream of the first flow control valve and the second and third flow control valves are all equal to the pump delivery pressure and the pressures downstream thereof are all equal to 200 bar, differential pressures across these flow control valves are equal to one another. The flow rate of the hydraulic fluid delivered from the first hydraulic pump is distributed in accordance with opening ratios of the first and second flow control valves, and the flow rate of the hydraulic fluid delivered from the second hydraulic pump is provided to the second actuator depending on an opening of the third flow control valve. As a result, the distributed flow rate of the hydraulic fluid from the first hydraulic pump is supplied to the first actuator via the first directional control valve, the distributed flow rate of the hydraulic fluid from the first hydraulic pump and the flow rate of the hydraulic fluid from the second hydraulic pump are joined and then supplied to the second actuator via the second directional control valve, thereby enabling the first and second actuators to be driven simultaneously.